Optical coherence tomography imaging system and method

ABSTRACT

An optical imaging system includes an optical radiation source ( 410, 510 ), a frequency clock module outputting frequency clock signals ( 420 ), an optical interferometer ( 430 ), a data acquisition (DAQ) device ( 440 ) triggered by the frequency clock signals, and a computer ( 450 ) to perform multi-dimensional optical imaging of the samples. The frequency clock signals are processed by software or hardware to produce a record containing frequency-time relationship of the optical radiation source ( 410, 510 ) to externally clock the sampling process of the DAQ device ( 440 ). The system may employ over-sampling and various digital signal processing methods to improve image quality. The system further includes multiple stages of routers ( 1418, 1425 ) connecting the light source ( 1410 ) with a plurality of interferometers ( 1420 a- 1420 n) and a DAQ system ( 1450 ) externally clocked by frequency clock signals to perform high-speed multi-channel optical imaging of samples.

This application is a continuation of U.S. application Ser. No.13/092,414, filed Apr. 22, 2011, which is a continuation-in-part of U.S.application Ser. No. 12/016,484, filed Jan. 18, 2008, which claims thebenefit of U.S. Application 60/885,874, filed Jan. 19, 2007 now expired,the contents of each of which are incorporated herein by reference.

INTRODUCTION

This application relates to a new OCT imaging system and methods forimproving the efficiency, speed, and quality of the acquisition,generating, and display of one dimensional or multi-dimensional OCTimages.

Optical coherence tomography (OCT) is an emerging imaging technologybased on low-coherence interferometry that enables non-invasive,cross-sectional imaging of a sample with micrometer scale resolution. Ithas been demonstrated that Fourier domain OCT (FD-OCT) techniques cansignificantly improve the sensitivity and imaging speed of an OCTsystem. In FD-OCT systems, the interference fringe signals are recordedas a function of optical frequency at high-speed using a broadband lightsource and a spectrometer pair, or a frequency swept source and adetector pair. After analyzing the interference fringe signals, thedepth-encoded reflectivity profiles of the sample are retrieved and usedto construct the OCT images.

A frequency swept source has been demonstrated to have many advantagesfor OCT imaging because it enables high efficiency detection ofback-reflected signals from the sample via a balanced detection scheme.Such high signal collection efficiency is essential for high-speeddetection of very weak signals reflected from deeper regions in asample. However, a swept source based OCT system has sonic drawbacks.First, because the scanning wavelength of a high-speed tunable laser isusually not linear in optical frequency space, recorded OCT data pointsmust be recalibrated from time domain to equally spaced data points inoptical frequency. A frequency clock module connected to the laser istypically used to provide the frequency clock signals of the laser asthe recalibration reference. This recalibration process can betime-consuming because it must be performed for each scan of the lasercorresponding to one axial line (A-scan) in the constructed OCT image,which greatly limits the real-time imaging speed of conventional OCTsystems. Second, in the dynamic process of actively tuning the laserfrequency, the laser undergoes significant changes in cavity conditions(i.e., cavity length, average mode number, or number of modes), whichcause both intensity and phase instabilities and noise in the laseroutput. The intensity noise and phase noise degrade detectionsensitivity and final image quality. This is a problem that cannot becompletely resolved by the balanced detection method.

SUMMARY OF THE INVENTION

This application discloses a method to improve OCT signals processingand imaging speed: The frequency clock signals of the source areprocessed to obtain a pulse train containing the relation of the opticalfrequencies of the source and the time. The pulse train is connected tothe external clock signals input of the data acquisition (DAQ) device.The DAQ converts the input OCT interference signals at each pulse in thepulse train and transfer the converted data points to computer memory.This operation mode of the DAQ system advantageously relieves the datatransfer load of the computer data bus and simplifies OCT signalprocessing. Embodiments of using both hardware and software methods toachieve this goal are disclosed.

Also disclosed are methods to improve OCT imaging quality: Byover-sampling the OCT signals and frequency clock signals, and applyingvarious algorithms to digitally process the over-sampled data points toimprove signal quality and reduce the amount of data points needed to betransferred to computer memory, the OCT image quality can besignificantly improved without compromising the imaging speed.

Additionally, disclosed are methods of computer processors controllingthe overall operation of the imaging system to employ parallel dataacquisition and signal processing routines. Parallel processing isimportant for real-time high-speed signal acquisition and imageconstruction because the computer is not idle while the DAQ fills thedata buffers.

A multiple-channel OCT imaging system that generates high quality OCTimages at high speed from multiple-channels simultaneously is alsodisclosed. The system employs multiple stages of routers to route (i)the optical output of the source to illuminate a plurality ofinterferometers, and (ii) the optical or electric output of theinterferometers to the detectors and DAQ system. In this manner, theimaging system can provide multiple OCT imaging channels for a single ora plurality of samples.

A real-time video-rate OCT microscope using swept source is demonstratedas an embodiment of the inventive system and method.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are particularly pointed out and distinctlyclaimed at the conclusion of the specification in the claims. Theforegoing and other objects, features and advantages of exemplaryembodiments of the invention will be apparent from the followingdetailed description taken in conjunction with the accompanyingdrawings.

FIG. 1 illustrates an exemplary embodiment of an optical coherenceimaging system with microscope sample interface;

FIG. 2 illustrates an exemplary temporal intensity profile of a sweptsource showing a forward and a backward scan;

FIG. 3 illustrates an exemplary hardware signal processing boardschematic according to an embodiment of the invention;

FIG. 4 illustrates an exemplary optical imaging system according to anembodiment of the invention;

FIG. 5 illustrates an exemplary optical imaging system according toanother embodiment of the invention;

FIGS. 6a and 6b illustrate exemplary embodiments of an interferometer ofan optical imaging system;

FIGS. 7a and 7b illustrate exemplary embodiments of a frequency clockmodule of an optical imaging system;

FIGS. 8-12 each illustrate exemplary embodiments of optical imagingsystems according to the principles of the invention;

FIG. 13 illustrates, in block diagram form, an exemplary method realizedin accordance with the principles of the present invention;

FIGS. 14-16 each illustrate exemplary embodiments of multiple-channeloptical imaging systems according to the principles of the invention.

FIG. 17a shows the frequency clock, signals and the clock pulse traingenerated from the piezo tuning VCSEL swept source with coherence lengthlonger than 20 mm.

FIG. 17b shows the frequency clock signals and the clock pulse trainfrom the piezo tuning Fabry Perot swept source with coherence lengthabout 8 mm.

FIG. 18a shows the OCT images from a tape sample for swept source withcoherence length longer than 20 mm.

FIG. 18b show the OCT images from a tape sample for swept source withcoherence length about 8 mm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure describes the best mode or modes of practicing theinvention as presently contemplated. This description is not intended tobe understood in a limiting sense, but provides an example of theinvention presented solely for illustrative purposes by reference to theaccompanying drawings to advise one of ordinary skill in the art of theadvantages and construction of the invention, in the various views ofthe drawings, like reference characters designate like or similar parts.

1. Principle of Swept Source OCT

In swept source Fourier domain OCT, a light source outputs opticalfrequencies as a function of time. The light is coupled to a sample anda reference reflector using an optical interferometer. Back reflected orback scattered light from different depths within the sample is combinedwith light from the reference reflector to form OCT interference fringesignals. After converted to electric signals by a photodiode detector,the OCT signals are digitized by a DAQ device to discrete digital datapoints. A Fourier transform is applied to the data points to detect theecho time delay and amplitude of the back reflected light from differentdepths within the sample and to construct cross-sectional images of thesample.

An interference signal detected by a single photodiode, as a function ofoptical frequency, is expressed as:

I _(PD)(ω)=I _(R)(ω)+2√{square root over (I _(R)(ω)I _(S)(ω))}cos(Δφ(ω))+I _(S)(ω)   (1)

where I_(R)(ω) and I_(S)(ω) are the optical frequency dependentintensities reflected from the reference and sample arms; Δφ(ω) is theoptical frequency dependent phase difference between the reference andsample arms; ω=ω(t) is the optical frequency sweep profile as a functionof time. The interference tern on the right hand side of Eq. (1) isexpressed as:

$\begin{matrix}{{I_{int}(\omega)} = {2{\sum\limits_{n}{\sqrt{{I_{R}(\omega)}{I_{n}(\omega)}}{\cos \left\lbrack {\frac{\omega}{c}z_{n}} \right\rbrack}}}}} & (2)\end{matrix}$

where I_(n)(ω) is the intensity of light reflected from the n-th layerin the sample and can be expressed as I_(n)(ω)=R_(n)(ω)I_(SS)(ω);R_(n)(ω) is the optical frequency dependent reflectivity from the n-thlayer; I_(SS)(ω) is the original spectrum of the swept source; and z_(n)is the depth of the n-th layer. It is supposed that the attenuation forthe reference arm light is uniform for all frequency components:I_(R)(ω)=μ²I_(SS)(ω), where μ² is the attenuation coefficient,

When a balanced detection scheme is employed in the interferometer,there is a 180-degree phase shift between the interference signals thatoccur in the two interference channels connected to the balanceddetector. Since the output from the balanced detector is the differencebetween the two input channels, the subtraction of these two signalsadds the second term but subtracts first and third terms in Eq. (1). Inthe ideal case where the two input channels of the balanced detector areperfectly balanced, the output from the balanced detector is given by:

$\begin{matrix}\begin{matrix}{I_{BD} = {4{\sum\limits_{n}{\sqrt{{I_{R}(\omega)}{I_{n}(\omega)}}{\cos \left\lbrack {\frac{\omega}{c}z_{n}} \right\rbrack}}}}} \\{= {4\mu \; {I_{SS}(\omega)}{\sum\limits_{n}{\sqrt{R_{n}(\omega)}{\cos \left\lbrack {\frac{\omega}{c}z_{n}} \right\rbrack}}}}}\end{matrix} & (3)\end{matrix}$

Eq. 3 reveals the fact that the optical reflectivity R_(n)(ω) from then-th layer z_(n) is linearly encoded in the frequency of the sinusoidalfunction

${\cos \left\lbrack {\frac{\omega}{c}z_{n}} \right\rbrack}.$

The deeper that the reflection occurs (corresponding to a larger z_(n)value), the higher the frequency in the detected interference signals.Applying a Fourier transform to the interference fringe signalsdecomposes the mixed signals into differentiate frequency components.The amplitude of the frequency component is in proportional to the lightreflectivity from a certain depth. Therefore, a complete depth profileof a sample can be measured by plotting the Fourier transformedamplitude as a function of frequency. When the incident beam performsanother one dimensional or two dimensional scans across the samplesurface, 2D or 3D OCT image data of the sample can be acquired anddisplayed.

The sweep profile ω=ω(t) is determined by the scanning mechanism (i.e.,sinusoidal) of the wavelength tuning component within the laser. Thenonlinear nature of this tuning curve requires that the resulting OCTsignals be recalibrated from equally spaced sample points in time toequally spaced sample points in frequency. A frequency clock module isused to monitor the output frequency of the laser, by generating aninterference fringe signals from a fixed delay in another opticalinterferometer. This calibration process can be performed by hardware orsoftware to linearly map the acquired OCT interference signal datapoints to optical frequency domain.

2. Methods and Systems 2.1. Optical Coherence Tomography Imaging System

FIG. 1 is a schematic of an embodiment of an optical coherencetomography imaging system using a microscope (MS) 100 as the sampleinterface. The light source (SS) 110 is, for example, a rapidly sweptexternal cavity laser with a wavelength sweep range from 1250 nm to 1400nm at scan frequency of 16 kHz (8 kHz forward and 8 kHz backward for antotal effective scan rate of 16 kHz). The typical 3 dB spectralbandwidth is measured to be >110 nm, and the typical average outputpower is 16 mW.

The main optical output of the laser is split by a 99:1 fiber coupler(FC) 120. One percent of the laser output is connected to a Mach-ZehnderInterferometer (MZI) 130 as the frequency clock module to producefrequency clock signals. The frequency clock signals can be processed bya signal processing board (SP) 140 to generate electrical pulses thatare equally spaced in optical frequency. The frequency clock signals canalso be processed by software, after digitization of the signals, togenerate a digital record containing the frequency-time relationship ofthe laser. The frequency clock signal forms the basis of therecalibration routine and will be described in detail below. Another onepercent of the laser output is tapped to record the temporal intensityprofile of the source.

The rest of the main laser output is routed to a fiber based Michelsoninterferometer through another fiber coupler (FC). In the reference armof the interferometer, the beam exiting from the fiber is collimated incollimator (C) 150 and reflected by a stationary mirror (M) 160 backinto the fiber. A manual or electronically controlled variableattenuator (VA) 170 is used to adjust the reference power to a properlevel for better detection sensitivity. In the sample arm, the fiber 180is connected to a microscope head, and the beam exiting from the fiberis also collimated and directed by a XY scanner (SD) 190 toward thesample. The sample is placed on an XY translation stage mounted on themicroscope. An infinity corrected long working distance objective (OBJ)is used for focusing the beam onto the sample. The long working distanceof the objective provides a large clearance (>20 mm) between the opticsand the sample, which enables easy handling of the sample. A 45°incidence cold mirror is inserted into the beam path to reflect thevisible light from the sample onto a CCD camera that recordsconventional video microscope images of the sample. An aiming laser 200centered at 632 nm (which can be seen by a human eye), or at 780 nm(which can be detected by the CCD camera), is coupled to the sample armof the interferometer to indicate the laser scanning position on thesample. A balanced detector (BD) 210 (i.e., PDB140C, Thorlabs®, Inc.)with 3 dB cut-off frequency of 15 MHz, optimized for low DC-offset (<1mV) and high impedance gain (>180,000 V/A), is used to record theinterference fringe signals in the Michelson interferometer. Theinterference fringe signals are connected to a signal processing board(SP) 140. The signal processing board processes the output of the MZIfrequency clock 130 signals to generate a pulse train with equal spacingin optical frequencies. A 14-bit digitizer is configured in externalclock mode to use the pulse train as trigger signals to sample the OCTsignals. The digitized data points are equally spaced in opticalfrequency so no additional frequency recalibration is needed. FourierTransform is applied to the data points and generates thedepth-dependent reflectivity profile of the sample. The computer alsogenerates waveforms through an analog output board to control the XYbeam scanning in the microscope head, to perform 2D or 3D OCT imaging ofthe sample. Also shown in FIG. 1 is optical detector D.

2.2 Swept Source

The light source 110 may be a swept source used in the imaging system issimilar to that described in U.S. Patent Publication No. 2006/0203859.Briefly, the swept source includes a semiconductor gain chip with onepartial-reflection coated facet that serves as the output coupler of thelaser, and another with anti-reflection (AR) coated facet toward theintra-cavity. The beam exiting from the AR coated facet of the chip iscollimated by an aspheric lens to illuminate a diffraction grating. Thelight diffracted by the grating is collected by an achromatic doubletlens and focused onto a highly-reflective mirror covered by a 10-μmslit. The grating is mounted on a resonant galvanometer scanner thatrotates about its axis. When the grating is rotating, the slit provideswavelength selection and feedback of the selected wavelength into thecavity, thus enabling high-speed sweeping of the output opticalfrequency of the laser. The measured temporal intensity profile of theswept source for a forward and a backward scan is shown in FIG. 2. Thetemporal intensity profile contains noise which is from the residueinterference effects within the laser cavity such as the etalon effectof the semiconductor gain chip. This interference signal can be used asthe frequency clock signal of the source since it is intrinsic in thelaser output and used to externally clock the acquisition of OCTsignals.

2.3 Frequency Clock Module

In Fourier domain OCT, as required by Eq. (3), the OCT signals must bere-sampled into linear frequency space, so adjacent data points have anequal optical frequency interval. Fourier transform can then be appliedto accurately recover the depth-dependent sample reflectivityinformation. The photo-detectors detect the interference fringes as afunction of time. However, because the optical frequency sweep of thelaser is determined by its sweep mechanism, or through application ofexternal driving signals, the resulting sweep of the laser frequencyoutput is typically not linear in time and simple sampling of thedetector signal using a fixed time base results in significantlydegraded image quality. Therefore, a frequency reference or a digitalrecord containing the time-frequency relationship of the laser must beestablished prior to the recalibration process. The recalibrationprocess maps the OCT data points to equal spacing in optical frequencydomain. Applying Fourier transforms to the recalibrated OCT data pointsyields the depth profiles of the samples.

A Mach-Zehnder Interferometer (MZI) is used as the frequency clockmodule. The MZI can have a fixed delay between its two arms or have atranslation stage to control the path length difference d between thetwo arms. Changing d changes the frequency of the resulting MZI clocksignal. Another balanced detection detector (for example, a Thorlabs®PDB120C, 80 MHz) is used to record the interference fringes of the MZI.The output of the MZI balanced detector is a sinusoid wave similar toEq. 3 and can be expressed as below:

$\begin{matrix}{I_{MZI} = {4{I_{SS}(\omega)}{\cos \left\lbrack {\frac{\omega}{c} \cdot d} \right\rbrack}}} & (4)\end{matrix}$

Although ω(t) is usually not linear in time, all the maximas andzero-crossings (4 points per MZI fringe cycle) in a signal measured as afunction of time are equally spaced in frequency. The Free-SpectralRange of the MZI clock is given by:

FSR_(MZI) =c/d   (5)

The number of MZI fringe cycles per laser swept is given by:

$\begin{matrix}{N_{MZI\_ fringes} = \frac{\left( {\omega_{\max} - \omega_{\min}} \right) \cdot d}{2{\pi \cdot c}}} & (6)\end{matrix}$

where ω_(max) and ω_(min) are the maximum and minimum angularfrequencies of the swept source. For a wavelength scanning range of 1240nm to 1380 nm, when the delay d is set at 6 mm, the FSR is approximately50 GHz and one scan of the laser generates approximately 480 fringecycles. If two data points are taken per MZI cycle, a total number of˜1000 points can be generated as the frequency reference for samplingthe OCT interference fringe signals.

Since the MZI delay can be continuously adjustable, and the number ofdata points per MZI cycle can be 2 or 4 or other numbers, the MZI clockmodule is very flexible in generating the required number of frequencyreference data points tier swept source OCT applications. Applying thebalanced detection of the MZI clock signal is a very effective method toremove the DC term in the detected signal and double the contrast of theinterference fringe signals.

2.4 OCT Data Acquisition Externally Clocked by Frequency Clock Signals

The frequency clock signals can be processed by software or hardware toexternally clock the acquisition of OCT signals. In conventional SS-OCTsystems, the OCT signals are recorded simultaneously with the clocksignals by a two channel high-speed digitizer. A software algorithmanalyzes the clock signals to build a digital record containing thefrequency-time relationship for every laser scan. This digital record isthen used to recalibrate the acquired OCT signal into linear frequencyspace. This approach requires sampling both the OCT data channel andfrequency clock signal channel. Since the laser can be scanning at avery high speed, the data transfer load from the DAQ device to thecomputer memory is very large, often exceeding the data transferbandwidth of the data bus (i.e., PCI bus). The data bus bandwidth limitsthe maximum data that can be processed by the computer processor and theOCT system imaging speed.

In view of the problems of the prior art, an embodiment of the presentinvention includes an OCT system, which uses the frequency clock signalto externally clock the DAQ device to sample the OCT interferencesignals. This may advantageously reduce the amount of data needed to betransferred from the DAQ device to the computer memory. The triggeringmechanism can be an electrical pulse train input to the DAQ device, or acopy of digital record residing in or uploaded to the DAQ board. Theelectrical pulse train and the digital record are generated by hardwareor software processing of the frequency clock signal. Using this method,the recalibration process of the OCT signals is done in the DAQ deviceand only the data points required for OCT image construction aretransferred via the computer data bus. Certain experimental systemsshowed a decrease of the data transfer load of the data bus by a factorof at least 2-5. in addition, the recalibration process is moved fromthe computer processor to the DAQ device which alleviates thecomputation load of the computer processor to allow the processor moreduty cycle for performing other tasks like Fourier transform, logarithmcalculation and display of the multi-dimensional image data.

This method is very useful for reducing the data transfer load inhigh-speed OCT imaging systems, since the output frequency of the laseris usually nonlinear in time. This method can be also applied to otherapplications requiring, for example, high speed analog to digitalconversion, high density sampling of raw signals, and a nonlinear timebase. Uploading a digital record, which contains the nonlinear time baseinformation and can be dynamically modified, to the DAQ device allowsthe DAQ device to use this digital record to externally clock the dataacquisition processes for all signal channels input to the DAQ device.Only the data points that are in a particular relationship with thedigital record are processed and transferred to memory devices, whileother redundant data points are discarded or not transferred. Thismethod requires less bandwidth and less time for transferring the datapoints, shares some data processing load of the main processor, thusadvantageously improves the speed and efficiency for the applications.

Another embodiment of the invention provides a hardware method foraccelerating the recalibration of OCT signals from time to frequencyspace using a signal processing board. The signal processing boardprocesses the frequency clock signals to generate clock pulses thatindicate when the output optical frequencies of the source have a linearrelationship. The digitizer is configured in external clock mode, andthe clock pulses output from the signal processing board are connectedto the external clock input of the digitizer to serve as the time basefor the analog to digital conversion of the OCT signal channel. In thismode, the OCT signals are digitized into data points with equal spacingin the optical frequency domain, ready for Fourier transform to generatethe depth profiles of the sample and construct OCT images.

FIG. 3 is an illustrative schematic of an example SS-OCT signalprocessing board. The signal processing board 300 generates pulse trainswith the pulses indicate when the output optical frequencies of thesource have a linear relationship. The pulse trains are connected to theexternal clock input of the DAQ card, to serve as the time base tosynchronize the data acquisition of OCT signals. In the example of FIG.3, fiber couplers 307 are used to split the light from a swept source305. A small portion (˜1%) of the laser output P_(REF) from swept source305 is monitored by an on-board optical detector 310 (typically aphotodiode). The detection bandwidth is chosen to be higher than themaximum frequencies generated by the MZI clock 315 to prevent signalaliasing. The laser scanning frequency signal from the MZI clock P_(CLK)from the output of the balanced detector 1 316 is divided by P_(REF) atDivider 1 320. This division normalizes the measurement data to the timedependent laser power curve. A 2.5-7.0 MHz band-pass filter 325 designedfor a 20 kHz scanning laser is used for bandpass filtering the frequencyclock signal. The bandpass filter rejects the low frequency and highfrequency noises on the clock signals and reduces the errors from thedecision circuit 330 in later stages of the signal processing board. Thedecision circuit 330 is a fast voltage comparator with a referencevoltage setting at zero, so all the zero-crossings in the clock signalsare converted to digital pulses indicating the time when the outputoptical frequencies of the source have a linear relationship. If morevalid pulses for sampling the OCT interference fringes are required, afrequency doubling circuit is used. A differentiator 335 generates thefirst order derivative of the clock signal and converts all the peaks tozero-crossings. Another voltage comparator 337 receives the output fromthe differentiator and generates a second set of digital pulses. The twosets of digital clock pulses are phase shifted by 90 degrees and arecombined by XOR digital logic circuits 340 into frequency doubled clockpulses. Depending on the XORs logic design, 2 or 4 pulses per MZI fringecycle can be generated. The original or the frequency doubled clockpulses are connected to the external clock input of DAQ device 350 toexternally clock the analog to digital conversion process. In the caseof 2 pulses per MZI fringe cycle, the clock pulses have a typicalfrequency range from 4-14 MHz when a swept source as described insection 2.2 is used; other sources may have significantly differentclock frequencies. This method can be adapted to provide a higher numberof clock pulses per MZI fringe cycle if required. As shown in FIG. 3, abalanced detector 360 couples the interference fringe signals from theinterferometer 365 to the OCT clock board 300.

2.5 Parallel Computing

In an example embodiment software configures the data acquisitionroutine and signal processing routine in parallel. The software controlsthe DAQ device to start the data acquisition routine; and withoutwaiting for the data acquisition routine to be finished, the softwarestarts the signal processing routine to process the previously acquireddata stored in memory; the software then checks the data acquisitionstatus after processing a certain amount of previously acquired data.The flow chart shown in FIG. 13 illustrates the principles of the imageconstruction routine.

2.6 Signal Enhancement by Over-Sampling OCT and Frequency Clock Signals

Further disclosed is a method to enhance signal and image quality in anOCT imaging system. The method includes over-sampling the OCT signalsand frequency clock signals at a sampling density higher than requiredby Nyquist sampling theory and utilizing the on-board processing powerof the DAQ device or computer processor to process the over-sampled datapoints according to the processed frequency clock signals—a digitalrecord that contains the frequency-time relationship of the source.Various signal processing algorithms can be applied to significantlyimprove signal strength, image contrast, and image quality.

Over-sampling the OCT signals increases the amount of data points thatcan be processed to produce images. Since many balanced detectorsconvert the received photons into electric current, and atrans-impedance gain module inside the balanced detector converts thecurrent into voltage output, for a continuous wave (CW) light sourcelike a swept source or a super luminescent diode, sampling the OCTsignals at a higher density means better photon detection efficiency andbetter system sensitivity. The recalibration process picks out the datapoints that are linearly spaced in optical frequency domain and throwsout other data points that are still valid points representing theoptical interference fringe signals. By using, for example, speciallydesigned algorithms to process the over-sampled data points andsalvaging the photons that are discarded in conventional OCT signalprocessing methods, the OCT signal strength can be increased,signal-to-noise can be enhanced, and the final image quality can besignificantly improved.

Over-sampling of the frequency clock signals produces more data pointsto represent the raw signals, thus the frequency-time relationship ofthe laser scans can be measured more precisely. An accuratefrequency-time relationship of the laser is critical for recalibrationof the raw OCT signals from time into optical frequency space. As aresult of over-sampling the frequency clock signals, the resolution andsignal-to-noise ratio of OCT images can be significantly improved, andsome imaging artifacts are reduced or totally removed.

The algorithms that can be used for processing over-sampled OCT signalsinclude, for example:

-   1. Multiple data points are averaged to be one data point according    to a digital record containing the frequency-time relationship of    the source; Fourier transform is applied to the averaged data points    to construct a depth profile of the sample. Alternatively, multiple    sets of the OCT data points are generated from the over-sampled OCT    data points according to multiple sets of digital records containing    the frequency-time relationship of the same source; Fourier    transforms are applied to each set of OCT data points. The outputs    of Fourier transforms of multiple sets of OCT data points are    averaged into one set to construct a depth profile of the sample.-   2. The over-sampled OCT data points are stored in memory and    compared with another set of over-sampled data points to improve    signal to noise ratio, enhance image resolution or contrast, or    enhance phase, polarization and spectrum information. The other    over-sampled data points used for the comparison can be acquired    from previous scans of the laser, or from other signal channels    acquired simultaneously with a current channel, or from a    pre-calculated data set stored in the memory device.-   3. The over-sampled OCT data points are averaged according to a    digital record containing the frequency-time relationship of the    source. A Fourier transform of the averaged OCT data points    generates intensity and phase information. The intensity information    is averaged to construct a depth profile of the sample. The phase    information is averaged to provide information about sample position    and motion, or various sample properties to the incident light    conditions. The various sample properties include, for example,    optical birefringence, absorption, fluorescence emission spectrum,    optical harmonic generation, and other linear or nonlinear optical    properties of the sample.-   4. The over-sampled OCT data points are averaged according to the    processed frequency clock signals. A Fourier transform of the    averaged OCT data points generates intensity and phase information.    The intensity and phase information are compared with another data    set in the memory or acquired from another signal channel    simultaneously or non-simultaneously. The compared intensity    information is used to construct a depth profile of the sample. The    intensity information can be averaged and digitally interpolated to    improve the resolution in detecting of certain reflection layers in    the sample. The compared phase information is averaged to provide    highly sensitive information about, for example, sample position,    motion of particles in the sample, or various sample properties    under incident light conditions. The various sample properties, for    example, include optical birefringence, absorption, fluorescence    emission, optical harmonic generation, and other linear of nonlinear    optical properties of the sample.

2.7 Multiple-Channel OCT Imaging System

Also disclosed herein is an OCT imaging system for acquiring highquality image data from multiple samples simultaneously. The systememploys multiple stages of routers to multiplex a plurality of OCTimaging channels with one light source. Each light source has afrequency clock module to externally clock the DAQ device acquisition ofthe imaging channel that is illuminated by this light source. The DAQdevice has multiple input channels for the plurality of OCT imagingchannels and the image data from multiple channels is acquiredsimultaneously or by using time-multiplexing. In this multiple-channelOCT imaging system, the DAQ process is externally clocked by theprocessed frequency clock signals from the frequency clock moduleserving each light source, which results in very efficient dataacquisition and processing, and very high imaging speed. In thismultiple-channel OCT imaging system, the over-sampling methods of OCTsignals and frequency clock signals may also be applied to improve theOCT image.

3. Additional System and Method Embodiments

FIG. 4 is an embodiment of a system for performing optical imaging of asample. This system includes an optical radiation source shown as aswept source 410. The swept source 410 outputs its optical frequenciesas a function of time. A frequency clock module 420 monitors the outputoptical frequency of the swept source 410 and outputs frequency clocksignals. The system also includes an optical interferometer 430 whichreceives an output from the swept source 410 and an optical detector todetect the interference fringe signals from the interferometer andconvert them to analog electrical signals. The optical detector may bean embedded module in the optical interferometer 430 or external to anoptical interferometer (see FIG. 1 optical detector D). Although theoptical interference happens inside an optical interferometer with orwithout the optical detector. The system further includes a DAQ device440 to convert the analog electric signals to digital data points. Thedata acquisition is externally clocked by the frequency clock signals. Acomputer controls 450 the DAQ system, processing of the digital signals,and construction of the depth profiles, and multi-dimensional images 460of the sample.

FIG. 5 is an embodiment of a system for performing optical imaging of asample. The optical radiation source 510 has a broadband spectral outputand outputs all optical frequencies simultaneously. The broadband source510 illuminates an interferometer 530 and produces OCT interferencefringe signals in optical frequency domain. A digital record is used asthe frequency clock signal from the frequency clock module 520 toexternally clock the data acquisition of OCT signals from aspectrometer. A computer 550 controls the DAQ system 440, processing ofthe digital signals, and construction of the depth profiles, andmulti-dimensional images 560 of the sample. The digital record isgenerated from an optical wavelength meter or an optical spectrometer asthe frequency clock module 520 of the light source.

As shown in FIG. 6a , a Michelson interferometer may include an opticalpath leading to a reference optical reflector 610, an optical pathleading to a sample 620 to be imaged, and an optical path where thelight from the sample and the reflector interfere to produceinterference fringe signals. In addition as shown in FIG. 6b , theoptical path leading to the reference reflector 630 and optical pathleading to the sample 640 can share a same optical path, forming acommon path interferometer. The reference reflector in either theMichelson type or common path type or any other type interferometers isa fixed single reflection surface, or has multiple reflection surfacesfrom known depths, or the position of the reflector is programmable.

The frequency clock modules shown in FIGS. 7a and 7b may use a fixed orvariable delay optical interferometer to monitor the output frequency ofthe source. Thus, FIG. 7a , for example, shows a variable delayMach-Zehnder interferometer (MZI), while FIG. 7b depicts a fixed delayMZI frequency clock. The MZI has a balanced output to suppress thecommon mode signals and enhance the interference fringe signals. Otherembodiments of the frequency clock module include an opticalinterferometer with at least one known delay that is fixed or variable,a wavelength meter or an optical spectrometer, or the light source andthe optical interferometer used in the same imaging system that havesome residue interference effects within them.

In another alternative embodiment of an optical imaging system, asillustrated in FIG. 8, the frequency clock signals from the frequencyclock module 815 are processed by an electronic signal processing board810 to produce a pulse train 820 indicating a relationship between anoutput frequency of the source and time. As shown in FIG. 8, the signalprocessing board 810 processes the frequency clock signals to produce apulse train 820. The pulses in the pulse train 820 indicate when theoutput optical frequencies of the source have a linear relationship. Thepulse train 820 is connected to the external clock input of the DAQdevice 840 to externally clock the DAQ system 840 to sample the OCTsignals from the interferometer 830.

FIG. 9 is an embodiment of a system for performing optical imaging of asample. In this embodiment the DAQ device 910 processes the frequencyclock signals to generate a digital record 920 containing thefrequency-time relationship information of the source. The digitalrecord is updated after every source sweep or after a certain number ofsource sweeps, and is used as a reference to control the sampling of theOCT signals coming from the interferometer 930.

FIG. 10 is an embodiment of a system for performing optical imaging of asample. In this embodiment the digital record 1010 containing thefrequency-time relationship of the source is generated by the softwareprocessing, for example in computer 1050, of the frequency clock signalsafter the signals are digitized by the DAQ device 1040 and transferredto computer memory. The digital record is uploaded to the DAQ device1040. Using the uploaded digital record as a reference, the DAQ device1040 acquires the OCT signals and selectively transfers the sampled datapoints to PC memory or disk files which are accessible to the software.The DAQ device 1040 is configured to use a fixed frequency samplingclock that is either internal or external to the device 1040.

In another embodiment of the imaging system as shown in FIG. 11, thedigital record 1110 containing the frequency-time relationship of thesource is saved in a memory device 1120 either as in the computer memoryor as in a disk file. The digital record 1110 is loaded to the DAQdevice 1130 and stored in an on-board memory buffer of the DAQ device1130, or in other programmable data or code buffers of the DAQ device1130. The DAQ device 1130 acquires the OCT signals and selectivelytransfers the data points to computer memory 1140 for furtherprocessing.

In another alternative embodiment of the imaging system as shown in FIG.12, a DAQ device 1210, memory devices 1220 and processors 1230 areembedded in one compact module or integrated into one board to form anembedded computing system 1200, or a single board computer system 1200.The DAQ device 1210 also can be a stand-alone device in communicationwith the embedded computing system 1200 or is a plug-in device installedinside a computer as a standard computer system configuration. Thecomputing system 1200 has a number of memory devices 1220 that areaccessible to both the DAQ device 1210 and the processor 1230 which iscontrolled by software. The software controls the data acquisitionprocess of the DAQ device 1210 which is externally clocked by thefrequency clock signals of the light source 1250. The software processesthe data points transferred from the DAQ device 1210 to memory andgenerates multi-dimensional OCT images 1260. The software also providesan operational interface for a user to control the imaging system.

In an embodiment illustrative of a method according to the principles ofthe invention as shown in FIG. 13, the data acquisition routine 1310 andsignal processing routine 1320 are configured in parallel. For example,a DAQ device starts the data acquisition routine 1310; and, withoutwaiting for the data acquisition routine 1310 to be finished, the signalprocessing routine 1320, to process the previously acquired data storedin memory, is started. The status of both routines 1310 and 1320 ischecked. As shown in block 1330, when the routines 1310 and 1320 arefinished the method proceeds to constructing a new OCT image 1300. Themethod may be executed by hardware, software or a combination thereof.The software may be embodied in a computer readable medium, which, whenexecuted by a processor or control system, causes the method to beexecuted.

In the embodiment shown in FIG. 14, a system provides optical images ofsamples 1440 a-1440 n using multiple imaging channels. This systemincludes an optical radiation source 1410, such as a swept source. Thesystem also includes a frequency clock module 1415 monitoring the outputoptical frequency of the source 1410. A plurality of opticalinterferometer modules 1420 a-1420 n are also provided. A first router1418 distributes the output power of the optical source 1410 to theplurality of interferometers 1420 a-1420 n and a second router 1425distributes outputs of the plurality of interferometers 1420 a-1420 n toa data acquisition (DAQ) system 1450. The DAQ system 1450 converts theOCT signals from the plurality of interferometers 1420 a-1420 n to OCTdata points, triggered by the frequency clock signals. Over-sampling ofthe OCT signals improves the signal quality and OCT image quality. Thesoftware executed by computer 1455 processes the OCT data points andgenerates multi-dimensional OCT images 1460 for multiple imagingchannels. The software also provides an operation interface 1460 for theuser to control the imaging system. If the DAQ system 1450 has asufficient number of sampling channels for the all the interferometers1420 a-1420 n, the second router 1425 is not required. The computer 1455also controls the probe control 1465 and sample probes 1430 a-1430 n.

The system of FIG. 14 may further includes an optical radiation source,a frequency clock module both having similar properties as discussedabove with regard to FIG. 5 (broadband source 510 and frequency clockmodule 520). The first stage of routers 1418 of FIG. 14 may be anoptical switching device that switches the output of the optical sourceamong input ports of the plurality of interferometers 1420 a-1420 n, oran M×N optical coupling device that couples the output of the opticalsource to multiple interferometer input ports. The plurality of opticalinterferometers 1420 a-1420 n of FIG. 14 is composed of multiple opticalinterferometers. Each optical interferometer may include a referenceoptical reflector, an optical path leading to the reflector, an opticalpath leading to a sample to be imaged which is labeled as sample probe1430 a-1430 n in this figure, and an optical path where the light fromthe sample and the reflector interfere to produce interference fringesignals. Each optical interferometer has similar properties as discussedabove with regard to FIG. 5.

In the embodiment shown in FIG. 15, the second router 1530 is an opticalswitching device that switches the optical interference fringe signalsfrom multiple interferometers 1420 a-1420 n to multiple opticaldetectors 1550, and the multiple optical detectors 1550 convert theoptical signals to electric signals, and the electric signals areconnected to the multi-channel DAQ device 1540. Alternatively, thesecond router 1530 can be an electric switching device that switches theOCT signals from multiple interferometers, when every interferometer hasits own detector to convert the optical interference signals to electricsignals, to the multi-channel DAQ device. In an example alternativeembodiment illustrated in FIG. 16, the first and second routers of FIG.15 can be the same optical switching device illustrated as first router1610 of FIG. 16. An optical beam splitter or an optical circulator 1620is used to direct the optical output of the interferometers to thedetector 1630 to convert the optical signals to electric signals. Theelectric signals are connected to the multi-channel DAQ device 1650.

In another embodiment of the system depicted in FIG. 14, the DAQ systemis composed of multiple DAQ devices with communication capabilitiesamong the DAQ devices. Each DAQ device has similar properties asdiscussed above with regard to FIG. 5. The data from different imagingchannels are either time encoded or data acquisition channel encoded. Asdescribed above a computer system and DAQ system may be a combination ofsoftware and hardware, integral or separate.

In a further embodiment of the system of FIG. 14, the data fromdifferent imaging channels are time encoded or DAQ channel encoded. Thetime encoded information allows the data of any channel be recoveredwhen the switching devices in the first and second routers are activatedto enable that particular channel. The DAQ channel encoded informationallows the data of any channel be recovered from a known hardwareconnection method.

The embodiment shown in FIG. 14 is demonstrated for a multi-channelimaging system based on one optical radiation source. This configurationcan be easily expanded to support multiple optical radiation sources ofsame center wavelength or different center wavelength in one system,with each optical radiation source requires its own frequency clockmodule.

4. External Clocking Versus Internal Clocking

A data acquisition device can be considered as a functional module thatrequires certain input and output signals to work. The input signalsinclude the analog signals from multiple channels which are the signalsto sample, the trigger signals which tell when to start the dataconversion process, and the clock signals which is the time-base used todetermine the rate to sample the data for all channels simultaneously.The output signals are the converted digital signals representing theinput analog signals.

Typically, a data acquisition device uses an internal sample clock todetermine the rate to sample all analog input channels. The internalsample clock is usually generated by an on-board clock circuit such as acrystal oscillator. The crystal oscillator generates fixed frequencypulse train signals with equal time spacing between adjacent pulses.When using this fixed frequency pulse train as the sample clock for theanalog to digital conversion process, the converted digital data pointsare also equally spaced in time.

However, there are occasions when using an external clock source isadvantageous, especially when sampling a signal where an irregularsampling period is necessary. It is important to configure the system sothat the incoming signal to be sampled is accompanied by an externalpulse train. The external pulse train acts as the external sample clock.The clock pulses have irregular time periods indicating the time toperform the analog to digital conversion process. Each pulse of thissignal is used to sample data for all channels simultaneously.

In swept source OCT imaging system, the Fourier domain OCT algorithmrequires the sampled data points of OCT interference fringe signals tobe equally spaced in optical frequency domain. However, due to manydifferent frequency tuning mechanisms used by swept sources, the outputfrequencies of the swept sources are usually not linear in time. Whenusing an internal clock to sample the OCT interference fringe signals,the sampled data points are linear in time but are not linear in opticalfrequency. Therefore, post signal processing steps such as numericalremapping process is needed to convert the sampled data points from timeto optical frequency domain. Please note the optical frequency has alinear relationship with the wave number.

Current technology (e.g., US Patent Application Publication2005/0171438) uses numerical remapping process to convert the sampleddata points from time to optical frequency domain. However, in order tocancel the distortion originating from the nonlinearities in the wavenumber function, the data has to be numerically remapped from uniformtime to uniform wave number space based on the wave number functionwhich is determined by a spectra calibration process. It is clear thatin the current system and method, the acquired data can be nonlinear inwave number and need the numerical remapping process. In order toperform the compensation, current technology requires the measurement ofthe laser output frequency function k(t) which is the relationship oflaser output frequencies as a function of time. The numerical remappingprocess is done by some sort of algorithm to convert the data from timeto optical frequency domain.

An embodiment of the invention provides the method that uses an externalcircuit board to process the frequency clock signals of the sweptsource, and produce a pulse train indicating the relationship of theoutput optical frequencies of the source and time. The pulse train isconnected to the external clock input of the DAQ device. The analog todigital data conversion happens exactly at the time that each pulseoccurs. Depending on the circuit board design, we can choose either therising edge or the falling edge of the pulses to externally clock thedata acquisition of all analog input channels simultaneously. Becausethe output frequency and time relationship of the source is contained inthe irregular spacing of the pulses in the pulse train, the swept sourceoutput frequencies are linear at each rising edge or falling edge of thepulses in the pulse train. Therefore the sampled data are equally spacedin optical frequency domain automatically. There is no need to performnumerical remapping of the data from uniform time to uniform wave numberspace. There is also no need to measure the laser output frequencyfunction k(t) which is required for the remapping process. The circuitboard can work with different types of swept sources without measuringtheir individual k(t) functions.

Therefore, in contrast to the existing approach, the disclosed method inthis disclosure does not require the remapping process because theacquired data are equally spaced in optical frequency domainautomatically; and it does not require the measurement of the functionk(t) of the swept sources either.

5. Long Coherent Length Swept Source

In one embodiment, this method is used for high speed, long range OCTimaging, in which a special swept source that supports long coherencelength at high sweep speed is employed. This method requires the sweptsource to support >20 mm long coherence length, >100 nm tuning range atsweep rate higher than 100,000 A-scans per second.

Long coherence length is an important specification in OCT imagingapplications utilizing the external clocking method. The relationshipbetween the coherence length and the average instantaneous line width ofthe swept source is given by

${L_{c} = {\frac{2\ln \; 2}{\pi}\frac{\lambda^{2}}{\Delta\lambda}}},$

where λ is the center wavelength of the swept source and Δλ is theinstantaneous line width of the source averaged over the whole sweep. Inexperiment, the coherence length of the swept source is measured as thepath length difference in the interferometer arms where the interferencefringe contrast drops to 50% of the fringe contrast at zero path lengthdifferent of the interferometer. Fourier domain OCT theory requires theOCT fringe signals to be sampled in optical frequency domain with evenfrequency spacing of the data points. The denser the sampling frequency(the smaller frequency spacing of sampled data points) the larger thedepth range can be measured and displayed on the OCT images. Longcoherence length of the swept source allows good interference fringecontrast at long delay of the interferometer. Good interference fringecontrast at long delays allows the electronics circuit to process thesignals and generate the very accurate pulse train containing enoughnumber of pulses to enable dense sampling of OCT fringe signals.

This disclosure describes the coherence length impact on OCT imagequality when applying the “external clocking” method on two sweptsources of different coherence length specs. The two swept sources arerunning at the same speed of 100,000 A-scans/second. The first sweptsource is a piezo tuning VCSEL swept source with coherence lengthmeasured longer than 20 mm, and the second swept source is piezo tuningFabry-Perot swept source with coherence length measured to be about 8mm. Both sources are connected to a 25 GHz frequency spacingMach-Zehnder interferometer to generate the frequency clock signals. Thefrequency clock signals are processed by an electronics circuit togenerate the digital pulse trains indicating the relationship of outputfrequency and time of the sources. The pulse train is connected to theexternal clock input channel of the DAQ device to externally clock thedata acquisition of OCT signals. The acquired OCT data is processed bycomputer software to generate OCT images. Except for the two differentswept sources used, all the other modules used in this experiment arethe same.

FIG. 17a shows the frequency clock signals and the clock pulse traingenerated from the piezo tuning VCSEL swept source with coherence lengthlonger than 20 mm, The 25 GHz frequency clock signals 1720 (green trace)has very good fringe contrast. The circuit board processes these fringesignals to generate the clock pulse train 1710 (yellow trace) with twopulses per fringe cycle precisely. When connecting this pulse train tothe external clock input of the DAQ card we obtain very good OCT imagesfrom a tape sample like shown in FIG. 18 a.

FIG. 17b shows the frequency clock signals and the clock pulse trainfrom the piezo tuning Fabry Perot swept source with coherence lengthabout 8 mm. The 25 GHz frequency clock signals 1740 (green trace) havelimited fringe contrast and large noise because of the limited 8 mmcoherence length of the laser. The circuit board processes these fringesignals to generate the clock pulse train 1730 (yellow trace) withmissing clock pulses where the fringe contrast reduces and noise becomeslarge. When connecting this pulse train to the external clock input ofthe DAQ card we obtain noisy OCT images from the same tape sample shownin FIG. 18b . Note the total imaging depth in the sample is also smallercompare to FIG. 18 a.

The above are some unique advantages of using external clock module toexternally clock OCT data acquisition with a special swept source thatsupports long coherence length at high sweep speed.

In an embodiment of the present invention, some or all of the methodcomponents are implemented as a computer executable code. Such acomputer executable code contains a plurality of computer instructionsthat result in the execution of the tasks disclosed herein. Suchcomputer executable code may be available as source code or in objectcode, and may be further comprised as part of for example, a portablememory device or downloaded from the Internet, or embodied on a programstorage unit or computer readable medium. The principles of the presentinvention may be implemented as a combination of hardware and softwareand because some of the constituent system components and methodsdepicted in the accompanying drawings may be implemented in software,the actual connections between the system components or the processfunction blocks may differ depending upon the manner in which thepresent invention is programmed.

The computer executable code may be uploaded to, and executed by, amachine comprising any suitable architecture. Preferably, the machine isimplemented on a computer platform having hardware such as one or morecentral processing units (“CPU”), a random access memory (“RAM”), andinput/output interfaces. The computer platform may also include anoperating system and microinstruction code. The various processes andfunctions described herein may be either part of the microinstructioncode or part of the application program, or any combination thereof,which may be executed by a CPU, whether or not such computer orprocessor is explicitly shown. In addition, various other peripheralunits may be connected to the computer platform such as an additionaldata storage unit and a printing unit.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing appropriate software. Other hardware, conventionaland/or custom, may also be included. When provided by a processor, thefunctions may be provided by a single dedicated processor, by a singleshared processor, by a multi-threaded single processor, by a singleprocessor with multiple processing cores, or by a plurality ofindividual processors, some of which may be shared. Explicit use of theterm “processor” or “controller” should not be construed to referexclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processorhardware, ROM, RAM, and non-volatile storage.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

What is claimed is:
 1. A system for optical imaging of samples withmultiple imaging channels, by using multiple stages of routers tomultiplex a plurality of OCT imaging channels with one light source intoone system, the system acquiring image data from multiple points on asample or from multiple samples, and image data acquisition isexternally clocked by frequency clock signals of light sources used inthe system, the system comprising: an optical radiation sourceoutputting optical frequencies as a function of time; a frequency clockmodule monitoring the output optical frequencies of the opticalradiation source, and outputting frequency clock signals; a signalprocessing circuit processing the frequency clock signals to produce apulse train indicating a relationship of the output optical frequenciesof the source and time; a data acquisition (DAQ) system adapted for ananalog to digital conversion process and externally clocked by the pulsetrain, wherein the pulse train is connected to an external clock inputof the DAQ system; a plurality of optical interferometer modules,wherein the interferometer modules are in connection with the opticalradiation source and the samples, and output interference signalscontaining information about the sample, wherein the interferencesignals are connected to input channels of the DAQ system; a first stageof routers distributing the output power of the source to the pluralityof optical interferometer modules, wherein the plurality of opticalinterferometer modules output interference signals containinginformation about the sample; a second stage of routers distributing theoutput of the plurality of optical interferometer modules to the DAQsystem; and a processor controlling the DAQ system, processing the dataoutput from the DAQ system and constructing multiple dimensional imagesof the sample for every imaging channel.
 2. The system as claimed inclaim 1, wherein the first router is an optical switching device thatswitches the output of the optical source among optical interferometermodules input ports, or the first router is an M×N optical couplingdevice that couples the output of the optical source to multiple opticalinterferometer modules input ports.
 3. The system as claimed in claim 1,wherein the second router is an optical switching device that switchesthe optical signals from multiple interferometers to multiple opticaldetectors, and multiple optical detectors convert the optical signals toelectric signals input to the DAQ system.
 4. The system as claimed inclaim 1, wherein the second router is an electric switching device thatswitches the OCT signals from multiple interferometers, when everyinterferometer has its own detector to convert the optical interferencesignals to electric signals, to the DAQ system.
 5. The system as claimedin claim 1, wherein the first and second router can be the same opticalswitching device, wherein an optical beam splitter or an opticalcirculator is used to separate the optical input and output of theinterferometers.
 6. The system as claimed in claim 1, wherein the DAQsystem comprises multiple DAQ devices with communication capabilitiesamong the DAQ devices.